What is the correct order of nitrification?

What is the correct order of nitrification?

• A. Nitrate → Nitrite → Ammonium
• B. Ammonium → Nitrate → Nitrite
• C. Nitrite → Ammonium → Nitrate
• D. Ammonium → Nitrite → Nitrate

Answer: (D)

Nitrification is the aerobic oxidation of reduced nitrogen in two steps. First, ammonium (NH4+) is oxidized to nitrite (NO2−) by ammonia-oxidizing bacteria. Then nitrite is oxidized to nitrate (NO3−) by nitrite-oxidizing bacteria. Since nitrite is the intermediate produced in the first step and consumed in the second, the sequence must be ammonium → nitrite → nitrate. In wastewater settings this two-step process is the standard, even though some organisms can perform both steps in one organism.

Nitrification is the aerobic oxidation of reduced nitrogen in two steps. First, ammonium (NH4+) is oxidized to nitrite (NO2−) by ammonia-oxidizing bacteria. Then nitrite is oxidized to nitrate (NO3−) by nitrite-oxidizing bacteria. Since nitrite is the intermediate produced in the first step and consumed in the second, the sequence must be ammonium → nitrite → nitrate. In wastewater settings this two-step process is the standard, even though some organisms can perform both steps in one organism.